Threshold potentials for fast kinetics during mediated redox catalysis of insulators in Li–O2 and Li–S batteries

Redox mediators could catalyse otherwise slow and energy-inefficient cycling of Li–S and Li–O2 batteries by shuttling electrons or holes between the electrode and the solid insulating storage materials. For mediators to work efficiently they need to oxidize the solid with fast kinetics but with the lowest possible overpotential. However, the dependence of kinetics and overpotential is unclear, which hinders informed improvement. Here, we find that when the redox potentials of mediators are tuned via, for example, Li+ concentration in the electrolyte, they exhibit distinct threshold potentials, where the kinetics accelerate several-fold within a range as small as 10 mV. This phenomenon is independent of types of mediator and electrolyte. The acceleration originates from the overpotentials required to activate fast Li+/e− extraction and the following chemical step at specific abundant surface facets. Efficient redox catalysis at insulating solids therefore requires careful consideration of the surface conditions of the storage materials and electrolyte-dependent redox potentials, which may be tuned by salt concentrations or solvents. Redox mediators catalyse the otherwise slow and energy-inefficient cycling of Li–S and Li–O2 batteries. An investigation of the kinetics of mediated Li2S and Li2O2 oxidations when the redox potentials of various mediators are tuned reveals threshold potentials for high reaction rates, which can be optimized in multiple ways.